Effects of Inbreeding on Rainbow Trout Populations

An approach for measuring inbreeding depression on the basis of differences between inbred and outbred half‐sib family groups is described. Using this method, estimates of inbreeding depression in populations of rainbow trout (Salmo gairdneri) following one generation of brother‐sister mating was calculated for three production traits–percent egg hatchability, percent fry survival from swim‐up to 150 days, and weight at 150 days. The inbreeding depression estimates obtained from male and female half‐sib families were 1.7 and 4.4% for percent egg hatchability, 22.2 and 16.1% for percent fry survival, and 12.0 and 9.8% for weight at 150 days, respectively. Estimates for fry survival and weight at 150 days were significant at P < 0.01 while egg hatchability estimates were nonsignificant. Following one generation of full‐sib mating, the calculated effect of inbreeding on the number of live fish and total weight of fish at 150 days of age showed a reduction of 16.1 and 24.4% in female families and 22.1 and 31.4% in male families, respectively. The difference between inbreeding depression estimates derived from male and female half‐sib groups provide a measure of the maternal influence. Generalized trends of increasing depression with increasing levels of inbreeding from F = 0.125 to F = 0.50 were significant for weight at all ages from 77 to 150 days, but were nonsignificant for percent hatchability or percent fry survival. Inbreeding depression estimates for weight are considered to be minimum estimates because higher mortality rates in the inbred families resulted in lower fish density during the rearing period and, therefore, compensatory growth which reduced the weight difference between inbred and outbred families.